This prospective study was performed at the Department of Infectious Diseases, University Medical Centre Ljubljana, Slovenia, in the period from 1991 to 2000. We included all adult patients with multiple erythema migrans who gave consent to lumbar puncture, had routine blood and CSF tests performed, and borrelial antibody titres in CSF and blood determined. In the majority of these patients skin, blood, and CSF specimens were cultured in MKP medium for the presence of Borrelia. Of 332 patients with multiple erythema migrans, 200 (115 females, 85 males, aged 15-80 years) fulfilled inclusion criteria. The median number of skin lesion was three (2-60). Sixty-three (31.5%) patients had no associated symptoms, whereas 137 (68.5%) patients (including two with arthritis, six with radicular pain, a patient with facial palsy, another patient with foot palsy and a patient with transitory diplopia) reported local and/or constitutional symptoms. Routine CSF examination revealed abnormal results in 62/200 (31%) patients: lymphocytic pleocytosis (6-1119 x 10(6)/L leukocytes) was found in 15 (7.5%) patients (six were clinically without systemic symptoms, six had mild systemic symptoms, three reported radicular pains) and elevated CSF protein concentration was present in 52 (26%) patients (nine also had elevated CSF cell counts). Intrathecal borrelial antibody production was demonstrated in eight (4%) patients (only three of them had elevated CSF cell counts) and B. burgdorferi sensu lato was isolated from skin lesions, blood, and CSF in 77/191 (40.3%), 3/154' (1.95%), and 2/200 (1%) patients, respectively. B. afzelii predominated among the isolates. In patients with multiple erythema migrans abnormal CSF findings are not rare and may be present without any clinical sign of central nervous system involvement.